StingRay is correct that C compilers produce code that is quite less good (although this depends a lot on the scope and how you choose your C statements) than kitten written assembly but it is supremely easy to transform C to assembly once you have prototyped your gameplay and settled on the game features.

"kitten written assembly"???!!!

What do animals have to do with programming?

For what it's worth, when I write code in ASM now the first thing I do is write pseudo-notes before I start coding the main loop, or even some of the subroutines. For example: